2. Dislocation Types

Introduction to Dislocations

• Dislocations are crucial for understanding the mechanical properties of materials.

• The theoretical strength of solids is derived from atomic bonding strength.

Theoretical Strength of Solids

• Derive theoretical maximum strength based on atomic bonding.

• Chart depicts bonding force as a function of interatomic distance.

  • Stable point exists: beyond this, atoms drift apart as force decreases.

• Maximum stretch of atomic bond is roughly one quarter of stable bond length (r0).

• Theoretical strength for most solids is about 1/15 of Young's modulus (E).

  • Yield strength / failure strength vs. Young's modulus plotted.

  • Ceramics, metals, and polymers analyzed.

  • Some ceramics (e.g., silica glass, diamond) approach theoretical limit.

  • Polymers can also approximate this limit at low Young’s modulus.

• Metals rarely achieve theoretical strength:

  • Metals are generally over an order of magnitude below theoretical strength.

• Conclusion: Dislocations are key to understanding why metals have lower specific strength than expected.

Atomic Lattice Defects

• Dislocations fall under atomic lattice defects that create tensile or compressive strains.

• Discussion of different atomic lattice defects:

  • Interstitial Impurities: Different atoms in non-lattice sites (defect A).

  • Self Interstitials: Same atom out of regular lattice position (defect C).

    • Create compressive strains due to additional atoms.

  • Vacancies: Missing atoms create extra space, leading to tensile strain (defect D).

  • Substitutional Impurities: Different atom in lattice position affecting strain based on size (defect H).

  • Precipitates: Groups of atoms forming within the lattice (defect E).

• Dislocations categorized as:

  • Edge Dislocations: Extra half-plane of atoms inserted (defect B).

  • Dislocation Loops: Extra half-planes terminate within the structure.

Edge Dislocations

• Edge dislocations cause two types of strain:

  • Tensile strain below dislocation line (atoms spaced apart).

  • Compressive strain above dislocation line (atoms packed tightly).

• /Movement of Edge Dislocations:

  • Applying shear stress can introduce extra half-plane defect.

  • Dislocation moves as atomic bonds switch during shear.

  • When dislocation exits, a step feature is created.

  • Creates plastic strain, which is non-recoverable after stress is removed.

Burgers Vector

• Burgers vector indicates the direction of material strain caused by dislocation:

  • Constructed by "walking" around a defect-free region.

  • Separation indicates the Burgers vector when an extra half-plane is present.

• Key characteristics:

  • Points in the direction of material displacement.

  • Exists wherever dislocation defects are present.

Screw Dislocations

• Difference from Edge Dislocations:

  • Dislocation moves in a direction perpendicular to applied shear stress.

• Visualized with a telephone book analogy during shear:

  • Core of the screw dislocation is the shearing front.

  • Dislocation motion is perpendicular to applied stress.

• Burgers Vector in Screw Dislocations:

  • Defined similarly but points in the direction of material displacement.

  • Orientation:

    • Burgers vector is parallel to the applied shear direction.

    • Dislocation line is also parallel to the Burgers vector.

• Comparison: Edge dislocation (Burgers vector perpendicular to dislocation line) vs. screw dislocation (Burgers vector parallel to dislocation line).

Conclusion

• Edge and screw dislocations lead to similar material displacement under stress, resulting in identical step defects at surfaces.

• Understanding these dislocations is crucial for material science and engineering.